1. Bacteria: An Introduction
Dr. Sinan Bahjat
M B Ch B, M Sc, F I B M S

2. Definition

3. Bacteria are microscopic organisms that is composed of a single cell
Bacteria are microscopic organisms that is composed of a single cell. These organisms can thrive in diverse environments (e.g. soil, ocean, and inside the human gut).

4. Human has a complex relationship with bacteria
Human has a complex relationship with bacteria. Sometimes they help us (as normal flora), while at other times they can be destructive, causing diseases.

5. Structure

6. Bacteria are prokaryotes
Bacteria are prokaryotes. The entire organism is consisting of a single cell with a simple internal structure.

7. Unlike eukaryotic DNA, which is present in the nucleus, the bacterial DNA presents free, in a thread- like mass called the nucleoid. 

8. A separate, circular pieces of DNA- which are called plasmids- are present also in the bacteria floating in cytoplasm.

9. Ribosomes are present also in both bacteria and eukaryotic cells
Ribosomes are present also in both bacteria and eukaryotic cells. These units are where proteins are synthesized from amino acids, using the information encoded in mRNA. 

10. Generally, bacterial cells are surrounded by two protective coverings: an outer cell wall and an inner cell membrane. However, some bacteria do not poses a cell wall at all.

11. on the other hand, some bacteria may have a capsule which is a third, outermost, protective layer.

12. Classification

13. A few different criteria are used for classifying bacteria
A few different criteria are used for classifying bacteria. The nature of their cell walls, their shape, and any differences in their genetic makeup; all are criteria for their classification. 

14. The Gram stain is a test that is used to identify bacteria through the composition of their cell walls. It is a technique discovered by Hans Christian Gram in 1884. 

15. Bacteria are first stained with crystal violet; which is a purple dye, which binds to a complex structure of amino acids and sugars found in the cell wall called peptidoglycan.

16. This is followed by a series of steps that finally remove any unbound or loosely bound crystal violet. 

17. Then the cells are stained with a second red- colored dye called safranin. 

18. Gram- positive bacteria show purple coloration because of the rich peptidoglycan in their cell wall. On the other hand, Gram-negative bacteria stain with red color.

19. The outer layer of lipids in the gram- negative bacteria does not bind strongly to crystal violet, thus, the dye is easily washed away during the staining process. 

20. As an example, Streptococcus mutans, that causes dental caries, is a Gram- positive bacterium, while Vibrio cholerae, that causes cholera, are Gram-negative bacteria. 

21. There are three basic bacterial shapes, Round bacteria are referred to as cocci; cylindrical, capsule- shaped bacteria as bacilli or rods; and spiral bacteria are called spirilla.

22. Thus, the classification criteria mentioned are based mainly on physiological properties and morphology. 

23. Bacteria in human health and diseases

24. Bacteria can be beneficial as well as being harmful to human health
Bacteria can be beneficial as well as being harmful to human health. Commensal bacteria (i.e. normal flora) tend to be helpful.  

25. The cells of normal flora present in the human body are estimated to be more than 10 times of human cells. The alimentary tract (i.e. the gut) have the highest number of microbial species as normal flora.

26. The human gut is a suitable place for bacteria, with lot of nutrients available for their sustenance.

27. The gut normal flora microorganisms are beneficial in digestion, they can compete colonization by harmful pathogens, and also, help in the development of the immune system. 

28. Other types of bacteria can be harmful and cause infections
Other types of bacteria can be harmful and cause infections. An example for these bacteria is the bacteria named: Streptococcus pneumoniae, which causes pneumonia. Also, S. Mutans that causes dental caries.

29. In oral cavity, more than 700 bacterial species are present, e. g
In oral cavity, more than 700 bacterial species are present, e.g.: Streptococci (like S. mutans), Actinomyces, Fusobacterium,… etc. All are part of the normal flora of the mouth, but it can cause opportunistic infection sometimes.

30. Finally, bacteria are important for our health, as the same time, they can be very dangerous when it cause diseases.

31. Thank You